1. Field of the Invention
The present invention relates to a switching power supply unit made up of a ringing choke converter in which a main switching element such as an oscillation transistor, etc., is connected to a primary winding of a transformer and a self-excitation oscillation takes place by positive feedback of the output from a bias winding of the transformer to the main switching element.
2. Description of the Related Art
In a ringing choke converter (hereinafter referred to as an RCC) in which a main switching element such as an oscillation transistor, etc., is connected to a primary winding of a transformer and an output is supplied to the secondary winding by self-oscillation, the oscillation frequency increases in a waiting state because of the following reason.
Because an RCC operates so that either a primary winding current or a secondary winding current may always flow, when the primary current and the secondary current are combined, the current seems to continuously flow. At a fixed load, that is, in the state of a rated load, etc., a conducting time period per unit time of the main switching element on the primary winding side is lengthened so as to be able to supply a fixed load current. However, in a waiting state in which no load or a small load is present, the electric power to be supplied to the load is extremely reduced, and accordingly the conducting time period per unit time of the main switching element is shortened so as to reduce the load current. Because of this, the switching frequency increases in a waiting state in the RCC.
On the other hand, in a general PWM switching power supply unit in which a main switching element connected to a primary winding of a transformer is made to operate as a separately excited oscillator, the oscillation frequency of the main switching element is constant regardless of whether the load is light or heavy. Then, the switching power supply unit is made to operate in a continuous current mode or in a discontinuous current mode in accordance with the condition of the load by controlling the pulse width. That is, the switching power supply unit is operated in a continuous current mode when the load is a fixed or variable load, and in a waiting state, the switching power supply unit is operated in a discontinuous current mode in which the current is made discontinuous by narrowing the pulse width in order to reduce the electric power to be supplied to the load.
However, in a switching power supply unit in which self-oscillation takes place, such as an RCC, there is a problem of decreasing efficiency because the oscillation frequency increases in a waiting state, and also in a switching power supply unit of a general PWM type having separate excitation oscillation, there is a problem of the decreasing efficiency in a waiting state as in the above RCC because the oscillation frequency is constant regardless of load size. Moreover, switching power supply units are disclosed in Japanese Unexamined Patent Application Publication No. 9-117134 and Japanese Patent No. 2956681. In these, the oscillation frequency is lowered by detecting the lowered power supply voltage of a control IC in a waiting state and the oscillation frequency is decreased by detecting the output lowered by an external signal in a waiting state, but both of these designs are based on a separate excitation oscillation. Accordingly the dissipation power of an IC and circuit for controlling the main switching element cannot be neglected, and particularly the dissipation power in a waiting state becomes large and not negligible and a substantial improvement of efficiency cannot be expected with these designs.
It is an object of the present invention to improve the efficiency in an RCC type switching power supply unit by lowering the oscillation frequency in a waiting state.
According to the invention, switching power supply unit comprises: a transformer, a main switching element, a rectifying smoothing circuit, a mode switching circuit and a delay circuit. The transformer has a primary winding, a secondary winding and a bias winding. The main switching element is connected to the primary winding and receives an output of the bias winding as a positive feedback so as to form a ringing choke converter having a self-excitation oscillation. The rectifying smoothing circuit is connected to the bias winding. The mode switching circuit is turned on and off depending on whether the rectifying smoothing voltage of the rectifying smoothing circuit is a threshold voltage or higher. The delay circuit is coupled to the mode switching circuit and is connected between the main switching element and the bias winding and delays the output of the bias winding and applies the delayed output to the main switching element. The delay circuit lengthens a turn-on delay time when the mode switching circuit detects an output voltage of the rectifying smoothing circuit which is less than the threshold voltage. The degree of coupling in the transformer may be adjusted so that the smoothing output voltage may be made less than a threshold voltage by cross regulation of the transformer when the load power equals a threshold level or less.
The switching power supply unit may have a plurality of winding outputs on the side of a secondary winding and the outputs may be connected.
Alternatively, an output voltage switching circuit for switching output voltages on the side of a secondary winding of a transformer may be provided.
According to the present invention, without using control ICs and complicated circuits for separate excitation oscillation, the oscillation frequency automatically decreases by making use of cross regulation of a transformer itself in a waiting state, and accordingly loss can be suppressed and by that much, the efficiency can be increased. Furthermore, dissipation power can be easily decreased in a waiting state by providing the secondary winding of a transformer with a switch.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.